We are studying the mechanism and assembly of helicases using x-ray crystallographic methods in order to gain insight into DNA replication and nuclear export, two pathways in which defects can cause cancer. In particular, through structural investigations of the helicase/primase from bacteriophage T7, I will address the mechanism of DNA unwinding to determine whether the hexameric T7 helicase/primase adopts a mechanism similar to the binding-change mechanism of F1-ATPase or a mechanism similar to dimeric helicases. I also expect to gain insight into assembly at the replication fork through the crystal structure determination of a complex between the T7 helicase/primase and single- stranded DNA binding protein. My structural studies on the mRNA export factor RAT8/Dbp5 focus on the protein-protein interactions that accompany the shuttling of RAT8/Dbp5 from the nucleus to the nuclear pore and finally to the cytoplasm. This will be accomplished through crystal structure determination of RAT8/Dbp5 alone and in complex with RAT7/Nup159, its interaction partner at the nuclear pore.